fix(android): reduce repeated work in getAllSupportedRegions

[higorlapacw]

Summary

• cache the digit regex used by maskNumber()
• reuse a single PhoneNumberUtil instance inside getAllSupportedRegions()
• format each example number once and reuse the result for both example values and masks

Why

getAllSupportedRegions() was doing avoidable repeated work for every supported region on Android:

• recompiling the same digit regex for every mask generation
• calling PhoneNumberUtil.getInstance() repeatedly inside the loop
• formatting the same example numbers multiple times before masking them

This keeps behavior the same while reducing allocations and duplicated work in the Android warm-up path.

Validation

• flutter pub get
• flutter analyze packages/flutter_libphonenumber_android

Refs #97